Authorities in Hong Kong have begun the unprecedented step of outlawing a political party that advocates the territory's independence from China.

Secretary for Security John Lee announced Tuesday that he had sent a letter to Andy Chan, the founder of the Hong Kong National Party, giving him 21 days to issue a rebuttal why the group should not be banned.

Lee said he was considering outlawing the party under an ordinance that cites the interest of national security and public safety, but did not give details of any particular accusations against the group. Under Hong Kong law, national security is defined as "safeguarding of the territorial integrity and the independence of the People's Republic of China."

"In Hong Kong, we have freedom of association, but that right is now without restriction," Lee told reporters.

This is the first time the Hong Kong government has sought to ban a political party since Britain returned sovereignty back to Beijing in 1997.

China established a "one country, two systems" formula that gave Hong Kong numerous freedoms, but Beijing has been tightening its grip in recent years, sparking a number of student-led pro-democracy and independence movements.

The pro-independence Hong Kong National Party was born out of the massive 2014 "Umbrella Revolution" street protests demanding fully free elections.